1. Field of the Invention
The invention relates to a method for picture-in-picture insertion, in which a sequence of inset pictures is written to a memory and is read from said memory for insertion into a sequence of main pictures, to be precise in accordance with the preamble of claim 1, and also to a circuit arrangement for picture-in-picture insertion, in particular for carrying out said method in accordance with the preamble of claim 3.
2. Description of the Related Art
Various methods and apparatuses for inserting one or more inset pictures into a main picture (PIP—Picture In Picture) are known. In this case, the inset pictures stored in the memory are read out synchronously with a main picture. Since the read-out speed is generally higher, in a manner corresponding to the decimation of the inset picture, than the write-in speed, a seam can occur in the inset picture on account of the write pointer being overtaken by the read pointer. In this case, the inset picture is composed of a current part and a preceding part, between which the seam runs.
In particular in the case where the two parts originate from different motion phases, a disturbing effect is produced since moving objects through which the seam passes are displayed in a distorted manner. Moreover, if the frequencies of the inset and main pictures do not correspond exactly, the seam drifts, which is perceived as particularly unpleasant.
Problems can arise in particular in the case where the main and inset pictures are present in different standards. For example, if the main picture is intended to be reproduced at a frame frequency of 60 Hz and the inset picture at a frame frequency of 50 Hz, the different raster positions cause periodic jumping of the picture at the differential frequency. For this case, in the known integrated picture-in-picture circuits, there is the possibility of changing to a so called fallback mode which is intended to avoid these disturbances. This mode is also referred to as field mode since only one of the two fields is written to the memory. This field is then displayed twice in each case, with the result that line doubling occurs in the representation. It does not matter which of these two fields is used.
Furthermore, this mode also enables a representation at a frame frequency of 100 Hz with minimal memory outlay, without continuous seam effects occurring. What is disadvantageous in this case, however, is that, in the field mode, a jump between the same rasters of different frames can occur, which causes a seam to be generated, although said seam becomes visible only in the case of moving pictures. A further disadvantage of this operating mode consists in the reduced vertical picture resolution.
In order to reduce the relative high memory requirement in the preferred frame mode, in many cases slight compression by a factor of between 0.5 and 0.8 is performed in particular in the case of relatively large pictures. One possible compression method is DPCM (differential pulse code modulation). In this method, the difference between successive pixels of a line is formed. This difference can be stored with a lower resolution, a non-linear quantization scheme being used as a basis.
In this case, however, the problem arises that the read pointer can overtake the write pointer during a line. In the event of picture storage without compression, in this case pixels are reproduced which belong chronologically to a preceding picture. In the case of compression, however, the content of such a line is mutilated since, after storage, decompression can be realized without errors only when the entire line content is consistent. The consequence of this is that when compression is employed, an error-free representation of both fields is possible only when no seam can occur. Consequently, in the case of picture signals with different standards or different frame frequencies, compression cannot be realized when the known reproduction methods are employed.